Heat-developable photographic materials are well-known and heat-developable photographic materials and processes for using such photographic materials are described, for example, in Shashin Kogaku no Kiso, Hi-Gin-En Shashin-Hen (The Elementary Course of Photographic Engineering, Section of Nonsilver Photography), pp. 242 to 255, Corona Publishing Co. (1982), and U.S. Pat. No. 4,500,626, etc.
In addition, a method for forming a color image by the coupling reaction of the oxidation product of a developing agent with a coupler is disclosed in U.S. Pat. Nos. 3,761,270 and 4,021,240. A method for forming a positive color image by a photosensitive silver dye bleach process is disclosed in U.S. Pat. No. 4,235,957.
Further, a method of imagewise releasing or forming diffusible dyes by heat development and transferring these diffusible dyes to a dye fixing element has been proposed. In this method, both a negative dye image and a positive dye image can be obtained by changing the kind of a dye-providing compound and the kind of silver halide to be used. Further details are disclosed in U.S. Pat. Nos. 4,500,626, 4,483,914, 4,503,137, 4,559,290, JP-A-58-149046, JP-A-60-133449, JP-A-59-218443, JP-A-61-238056 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")), EP-A-220746, Kokai Giho (JIII Journal of Technical Disclosure) No. 87-6199, EP-A-210660, etc.
Various methods have been proposed as to methods of obtaining positive color images by heat development. For example, a method in which a so-called DRR compound (a dye releasing redox compound) is converted to a compound of an oxidized form having no dye-releasing capability, and the compound, in the presence of a reducing agent or a precursor thereof, is heat developed, the reducing agent is oxidized corresponding to the exposure amount of silver halide by heat development, and the compound is reduced by the remaining reducing agent not oxidized to release a diffusible dye is disclosed in U.S. Pat. No. 4,559,290. Further, a heat-developable color photographic material using, as a compound which releases a diffusible dye by the same mechanism, a compound which releases a diffusible dye by the reductive cleavage of an N--X bond (wherein X represents an oxygen atom, a nitrogen atom or a sulfur atom) is disclosed in EP-A-220746 and Kokai Giho No. 87-6199 (No. 22, Vol. 12).
Heat-developable color photographic can be developed with a compact developing machine because development processing can be performed simply and rapidly as compared with ordinary wet developing photographic materials. Therefore, comparatively inexpensive color copiers and color printers of a system of silver salt color photographic material have been developed and now on the market. It has been thought that various improvements are necessary for further widening the use for these machines. Many kinds of exposure light sources are proposed for such a photographic material. For example, as a digital exposure light source, a light emitting diode (LED), a semiconductor laser (LD) and a variety of fluorescent substances are used at present.
Various plans have been devised hitherto, for example, with respect to inexpensive LED and LD, a plurality of elements are arranged in a row to perform scanning exposure to shorten exposure time. However, in such a method of using a plurality of elements, there arises a problem that density unevenness of an image occurs due to the scatter among elements. Further, the wavelengths of LED and LD fluctuate due to the heat generated by LED and LD themselves during use and the temperature variation by the heat generated from other parts of an exposure unit, and the density unevenness of an image also occurs by the variation of light wavelength. The variation of light wavelength occurs particularly conspicuously in inexpensive LED although it also occurs in LD.
When a plurality of light sources are used, the present inventors devised a means of measuring the individual light amount and wavelength in regard to the density change due to the nonuniformities of the light emitting wavelength and the light amount of the light source, and previously compensating them to become a uniform result. However, the density unevenness could not be improved sufficiently contrary to expectations. This was due to the fact that in the spectral sensitivity region wherein the sensitivity change to the wavelength was sudden, there were errors in the measurement of wavelength, hence compensation was insufficient. It is possible to select the same wavelength in advance, but this means is extremely low yielding and economically disadvantageous.
Further, the present inventors tried to perform sufficient temperature control to the wavelength fluctuation during use, but it was found that suppressing the wavelength fluctuation was difficult as LED and LD themselves generated heat and, in particular, the temperature had fallen before use of these units at rise time.
Moreover, when a plurality of light sources are used for exposure, exposure is performed with a plurality of light sources scanning at the same time as one block. In that case an individual light emitting element in one block is exposed simultaneously (without a time lag), but a gap between one block and the next block is to be exposed with taking time by the time of scanning. In the case where the difference is generated in sensitivity between the time when exposure is carried out without a time lag and the time when exposure is carried out with a time lag, the doubled (overlapped) part between blocks is to be seen differently from the double (overlap) of beams in one block. Although little as difference in density of an image, this exposure streak unevenness is easily observed visually, therefore, this is the item to which care must be taken when a plurality of light sources are used for exposure.
For shortening the entire imaging time, scanning exposure not only in one direction but on the way back, i.e., reciprocating writing, is conducted in many cases. In such a case, the exposure interval time at the doubled (overlapped) part between one block and the next block is not constant and the exposure interval time at the first double (overlap) and the last double (overlap) is varied, as a result exposure streak unevenness is more easily observed visually. This is required to be improved as reciprocating writing unevenness among the above exposure streak unevenness.